1. Technical Field
The present invention relates to a solid-state imaging device and a method of driving the same.
2. Related Art
Japanese Laid-Open Patent Publication No. H2-50480 discloses a solid-state imaging device which includes a photo-electric conversion unit formed in a semiconductor substrate and generating signal charge corresponding to incident light; a charge accumulation unit provided in the semiconductor substrate as being adjacent to the photo-electric conversion unit, and temporarily accumulating the signal charge; a transfer gate unit receiving the signal charge from the charge accumulation unit, and transferring it; and a CCD.
FIG. 6 shows a sectional view of the solid-state imaging device disclosed in Japanese laid-open patent publication No. H2-50480. In this configuration, an N-type region 12 is commonly formed over a region ranging from the photo-electric conversion unit, the charge accumulation unit 17, the transfer gate unit 18 and the CCD. In the surficial portion of the charge accumulation unit 17 in the substrate, a P+-type region 14 is formed as being connected to a surface P+-type region 4 of the photo-electric conversion unit, so as to form a p-n junction at a level shallower than the P+-type region 4 forms. Also a P−-type region 15 is formed in the surficial portion of the substrate under a transfer gate 8. Because the N-type region is formed thicker in the charge accumulation unit 17 than in the photo-electric conversion unit as shown in the drawing, the charge accumulation unit 17 has a deeper potential well.
FIG. 7 shows a sectional view of a solid-state imaging device disclosed in Japanese Laid-Open Patent Publication No. H6-236987.
The solid-state imaging device includes an impurity-diffused region 1002 (photo-electric conversion unit) converting the incident light into signal charge, and accumulating it; a readout region 1007 into which the signal charge accumulated in the impurity-diffused region 1002 is read out; and a charge readout gate 1008 controlling readout of the signal charge accumulated in the impurity-diffused region 1002 into the readout region 1007. Between the impurity-diffused region 1002 and the charge readout gate 1008, there is formed a high-concentration impurity-diffused region 1009 having a potential set lower than that of the impurity-diffused region 1002, and allowing, in an accumulation period, the signal charge to flow thereinto from the impurity-diffused region 1002 to be accumulated therein. An accumulation gate electrode 1011 is formed on the high-concentration impurity-diffused region 1009. The high-concentration impurity-diffused region 1009 has a different conductivity-type from that of the charge readout gate 1008.
In thus-configured solid-state imaging device, the signal charge is readout as described below. First, in the accumulation period of the signal charge, the accumulation gate electrode 1011 and the readout gate electrode 1003 are brought down to the low level. The signal charge herein is accumulated in the high-concentration impurity-diffused region 1009. When only the readout gate electrode 1003 is brought up to the high level, the signal charge accumulated in the high-concentration impurity-diffused region 1009 is allowed to flow into the charge readout gate 1008. If a negative voltage is then applied to the accumulation gate electrode 1011, the signal charge accumulated in the high-concentration impurity-diffused region 1009 is allowed to rapidly flow into the charge readout gate 1008.
The prior arts described in the above have, however, suffered from problems in the aspects below.
The solid-state imaging device described in Japanese Laid-Open Patent Publication No. H6-236987 has the charge readout gate 1008 composed of the surficial portion of the semiconductor substrate 1001 which falls between the high-concentration impurity-diffused region 1009 and the readout region 1007. In this configuration, the charge accumulated in the high-concentration impurity-diffused region 1009 is read out into the readout region 1007, as being transferred through a channel formed in the surficial portion of the semiconductor substrate. Therefore in the process of reading the charge accumulated in the high-concentration impurity-diffused region 1009 out into the readout region 1007, the charge readout gate 1008 can never have a deep potential unless the readout gate electrode 1003 is applied with a high voltage. In particular, the potential at the interface between the high-concentration impurity-diffused region 1009 and the charge readout gate 1008 will remain at a high level, and will degrade the transfer efficiency.
Similarly, the solid-state imaging device described in Japanese Laid-open patent publication No. H2-50480 has the transfer gate unit 18 composed of the P−-type region 15 having a conductivity type different from that (N-type) of the photo-electric conversion unit and the charge accumulation unit. Also in this configuration, it is therefore necessary, in readout of the charge from the charge accumulation unit 17, to apply a high voltage to the transfer gate 8.